MOS combustible gas sensors operate by catalytic oxidation of combustible gases. Substantial efforts have been expended in recent years towards the development of combustible gas sensors using semiconductor MOS technology.
Generally the MOS gas sensor consists of a semiconductor substrate with an ohmic contact on one side and with the other side covered by an insulating layer with a metal gate on top. The metal gate is composed of a metal capable of catalyzing the oxidation of combustible gases. As a result of catalytic redox reactions on the gate surface, certain atomic or molecular species are generated which can diffuse through the porous gate to the metal gate/insulator interface where they can ionize. These ions can penetrate through the insulator thereby changing the potential distribution across the device. This changes the potential of the insulator/semiconductor interface and thus the depletion layer inside the semiconductor which in turn shifts the voltage dependent admittance characteristic of the device.
A significant problem with such existing MOS gas sensors involves how to achieve deposition of a good quality metal contact on the back side of the semiconductor substrate and a catalytically effective and reliable metal gate. It has become apparent that existing deposition processes cannot achieve the necessary adherence required for reliable sensor performance. Previously used techniques, such as plasma sputtering or electron beam evaporation have produced unstable contacts due to a lack of sufficient adherence between the deposited metal and the insulator.
Prior art sensor structures, such as the structure disclosed by A. Baranzahi et al in Transducers 95 and Eurosensors IX, Vol. 1, Stockholm 1995, pp 74-44, attempted to solve the problem of adherence by depositing a buffer layer of another metal between the semiconductor substrate and the metal to be deposited thereon, for example titanium or tantalum. However, the presence of such metal buffer layers between the gate and insulator can affect the catalytic activity of the gate. In fact, during high temperatures, the buffer layer often diffuses into the metal gate, poisoning the catalyst, and thereby reducing the catalytic activity of the metal gate. High temperature operation is an inherent necessity for catalytic detection of hydrocarbons. There is thus a significant need for metal deposition methods that improve the adherence of the films to their substrates without the addition of buffer layers.